Metered dose inhalers (MDIs) were developed in the 1950s for treatment of certain respiratory conditions such as asthma, emphysema, bronchitis, and the like. MDIs have since become extremely popular for inpatient and outpatient treatment, due to their effectiveness, low comparative cost, portability, and ease of use. Their development has included the introductions of many different drugs and many different devices in which to use them. In the last fifteen years, MDIs have increasingly been used for mechanically ventilated patients, as opposed to the previous method of using compressed gas powered nebulizers to deliver aerosolized medications. Nebulizers are problematic due to the complexity and cost of associated equipment, difficult infection control, and labor costs.
An MDI is a small, pressurized canister containing multiple doses of medication in liquid form, a propellant, and other inert ingredients to stabilize the medication and prepare it for aerosolization. The canister also contains a metering valve, which precisely “meters” doses of medication prior to discharge. A spring loaded plunger discharges the measured dose of medication through an integral nozzle at the end of the canister, the medication then exiting the nozzle in a plume of very small particles. This plume is then actively inhaled by the patient, either through a MDI holder, an independently manufactured holding chamber or spacing device, or other mouthpiece. In the case of the mechanically ventilated patient, the MDI is discharged into an adapter in the inspiratory tubing of the ventilator or into a spacer placed in the inspiratory tubing of the ventilator. The ventilator cycled volume then carries the plume to the lungs of the patient under pressure via an artificial airway such as an endotracheal tube or tracheostomy tube. Some spacers or holding devices can be used interchangeably between intubated and non-intubated patients.
Heat and moisture exchangers (HME) are apparatus which passively collect heat and humidity exhaled by an intubated patient and passively return some of such heat and moisture to the patient in the subsequent breath. Since lungs and their conducting airways are reliant on moisture for their function, the lack of moisture for even a short period of time, such as a few hours, can be damaging to lung and airway tissues and can be potentially fatal. In the non-intubated patient, heat and moisture are supplied by the upper airway, including the nose, mouth, and oropharynx. The intubated patient has an endotracheal tube or tracheostomy tube passing through the upper airways, removing the patient's ability to add heat and moisture to incoming air. Air/oxygen mixtures delivered from mechanical ventilators are cold and virtually dry; conditioning the gas mixture with heat and moisture minimizes the loss of body heat and moisture through ventilation. An HME is a rigid walled apparatus with an inlet and an outlet connectable between a ventilator circuit and a tracheal tube or endotracheal tube and containing a medium which absorbs heat and moisture from a gas saturated with heat and moisture, such as gas exhaled by a patient; this medium releases heat and moisture to cooler and drier gas such as that being delivered by a mechanical ventilator. Some HMEs are referred to as hygroscopic condenser humidifiers (HCH). HCH use a chemically treated medium to absorb and release heat and moisture. In the scope of the present invention, a HCH is a form of an HME. The medium in an HME can be a simple sponge-like material, a chemically treated sponge-like material, a chemically treated paper or polypropylene material, or other such heat and moisture absorbing material. It also may contain metal or metallic fibers to increase heat exchanging properties. A great variety of heat and moisture exchanging media, or referred to interchangeably as HME medium, exists.
An HME is referred to as an HMEF with the addition of a filter, such that gas delivered by a ventilator is first passed through a filter medium integral in the HME.
An HME is placed between the distal end of a ventilator circuit and the proximal end of an endotracheal or tracheostomy tube; thus exhaled gas is partially re-breathed by the patient, and, along with the re-breathed gas and dry, incoming gas from a ventilator, some of the heat and moisture the patient previously exhaled.
In many institutions, HME are used for the first twenty-four hours or more of mechanical ventilation before switching to a conventional active heat and humidity system. The advantages of HME are that they require no complex humidity and heat generators, are low cost, and are effective in returning exhaled heat and moisture to the patient. For most patients using short term mechanical ventilation, HMEs are a cost effective alternative to expensive, electrically powered heat and moisture systems.
A heated humidifier (HH) is an active system for adding heat and moisture to ventilator circuits for intubated and non-intubated patients. An HH is typically mounted on a mechanical ventilator and is electrically powered. Gas exiting from the ventilator is passed through a HH chamber comprised of heated canister with or without an integral wick to increase efficiency. An HH can regulate the amount of moisture in the fluid in a mechanical ventilating circuit and can further regulate the temperature of the fluid passing through a mechanical ventilator circuit. HHs are bulky and are heated, such as electrically, and thus are not referred to as passive heat and moisture exchangers (HME).
MDI and nebulized medications cannot pass through HMEs, since the medium will block nearly all of the aerosol particles before they can be delivered to the airway. This will also eventually clog the HME. To avoid this, current practice requires that the HME be removed from the circuit and substituted with an MDI spacer or conduit adapter. After medication is delivered via the MDI spacer, the spacer or conduit is removed and the HME is replaced. This procedure may occur as often as every hour or two, or at longer periods such as every 12 hours or so. Each time the circuit is opened for this procedure, there is risk of biocontamination of the circuit, and thus, the patient's airways. This is documented to be a cause of ventilator associated pneumonia (VAP). Another potential problem with opening ventilator circuits is the loss of end-expiratory pressure, resulting in collapse of lung segments. In certain disease states, it is very difficult to re-inflate these segments; the result is loss of ventilated lung tissue and altered gas exchange; this can be life threatening.